1. Field of the Invention
The present invention relates to a method and an apparatus for continuously producing an artificial marble plate with a constant thickness.
2. Description of the Related Art As exemplary apparatuses for producing a hardened polymer plate by continuously polymerizing compounds, Japanese Patent Laid-open No. Hei 10-217264 discloses “a method and an apparatus for continuously producing an artificial marble palate made of acrylate resin”, and Japanese Patent Laid-open No. 2000-317957 discloses “a method and an apparatus for producing an artificial marble plate”.
FIG. 1 is a cross-sectional view of the continuous hardening apparatus disclosed by Japanese Patent Laid-open No. Hei 10-217264. The continuous hardening apparatus comprises a pair of endless steel belts 101, an idle pulley 102 serving as miscellaneous equipment thereof, a driving pulley 102′, upper and lower film feed units 103 and 104, a raw material compound feed unit 105, a thickness adjusting unit 106, hardening chambers 107, 108, 109 and 110, nozzle units 111 and 111′, a driving motor 112, and belt back-up rolls 113 and 113′. This apparatus is characterized in that a polymerized raw material compound for an artificial marble plate is continuously entered into the apparatus and then continuously hardened by means of the movement of the belts 101, thus producing a platy artificial marble semi-product, and the produced semi-product is produced as a final artificial marble plate by an edge-trimming step and a surface-polishing step. Here, each of the hardening chambers 107, 108, 109 and 110 is divided into two or more areas, and each of the divided areas includes an air-circulating unit having a blower, a heater, a circulating duct and a nozzle unit. The two divided areas of each of the hardening chambers 107, 108, 109 and 110 are symmetrically installed on the upper and lower surfaces of one pair of the endless steel belts 101 provided with the raw material compound stacked thereon, and provides a hot blast required to produce the final product by hardening the raw material compound, thus heating and cooling the upper and lower surfaces of the polymerized raw material compound.
The hot blast is transmitted into the raw material compound via the air-circulating units symmetrically installed on the upper and lower surfaces of the endless steel belts 101, thereby hardening the raw material compound. In this case, the heat from the hot blast (that is supplied from the air-circulating units disposed on the upper surface of the endless steel belts 101) is transmitted to the upper surface of the raw material compound by convection, while the heat from the hot blast (that is supplied from the air-circulating units disposed on the lower surface of the endless steel belts 101) is transmitted to the lower surface of the raw material compound by conduction since the hot blast is first transmitted to the endless steel belt and then to the raw material compound.
That is, since the technique disclosed by Japanese Patent Laid-open No. Hei 10-217264 applies different heat transmission manners to the upper and lower surfaces of the raw material compound, it is difficult to supply heat in the same amount to the upper and lower surfaces of the raw material compound, thus causing a warpage (stress torsion) on the artificial marble plate obtained by hardening the raw material compound.
FIG. 2 is a cross-sectional view of the apparatus for producing an artificial marble plate disclosed by Japanese Patent Laid-open No. 2000-317957. This apparatus comprises endless belts 201 and 201′, rotary drums 202, 202′, 203 and 203′, rollers 204 and 204′, a metering pump 205 for feeding a raw material 205, a raw material injection unit 206, gaskets 207 and 207′, hot water spray units 208 and 208′, second heating zones 209 and 209′, cooling zones 210 and 211, a belt driving motor 212, a first heating zone 213, heating units 214 and 214′, and a produced artificial marble plate 215. The apparatus of this patent uses two pairs of the endless belts 201 and 201′, and employs a hardening method in which a polymerized raw material for an artificial marble plate is hardened by spraying hot water to each of the endless belts 201 and 201′. Since the heat transmission to the upper and lower surfaces of the polymerized raw material compound is performed by the same manner, this hardening method prevents the warpage of the artificial marble plate obtained by hardening the raw material compound. However, the above hardening method requires a plurality of heating zones and cooling zones, thus complicating the structure of the apparatus itself.
Further, the above-described two apparatuses shown in FIGS. 1 and 2 must be large-sized so as to control the tensions of the one pair or two pairs of the steel belts and operate the steel belts.